As optical fiber communications systems proliferate, the problem of interconnecting optical components having unequal modal spot sizes assumes increasing importance. Such interconnections are required in a variety of circumstances including: 1) the interconnection of laser sources to fibers, 2) the interconnection of two fibers having dissimilar modal properties, and 3) the interconnection of fibers to waveguides and waveguides to fibers. Such interconnections have been an area of active research, and a variety of approaches have been developed. For example, pretapered performs have been prepared to draw tapered regions for connecting lasers to fibers. (see J. Armitay et al., J. Lightwave Technol. LT-5, 70, 1987). Fibers have been tapered through capillaries in order to achieve beam-expansion (see K. P. Jedrzejewski, 22 Electron. Lett. 106, 1986) and fiber cores have been thermally expanded for splicing dissimilar fibers (see S. G. Kosinski et al., Proc. Optical Fiber Communications Conference OFC, Paper Th 16, 231, 1992). These techniques, however, all depend on control of the physical dimensions of the fiber core--a control which is difficult and expensive to achieve.
Another technique for fabricating a waveguide structure that transforms an optical beam of a first modal spot size to a beam of a second modal spot size is disclosed in U.S. Pat. No. 5,416,863 to Vengsarkar. In Vengsarkar the modal spot size is varied by varying the refractive index differential between the core and cladding. Vengsarkar accomplishes this variation in the refractive index differential by irradiating the waveguide structure at a wavelength which is absorbed by photosensitive defects in the core material so that the refractive index differential increases. In particular, Vengsarkar increased the index differential in germanium-doped silica waveguides by irradiation with ultraviolet light at a wavelength of 247-248 nm. This wavelength corresponds to germania-related color center defects that are known to be present in the germanosilicate core material. In Vengsarkar, the photosensitivity of the germanium-doped core was enhanced by impregnating the core with molecular hydrogen. This technique is well known and is disclosed, for example, in R M. Arkins et al., "Mechanisms of Enhanced UV Photosensitivity Via Hydrogen Loading in Germanosilicate Glasses," Electron. Lett., vol. 29, p. 1234, 1993. However, similar to the limitations of the method disclosed by Hibino et al., Electron. Lett., vol. 29, pp. 621-623, 1993, which is discussed in co-pending application Ser. No. 08/396023, U.S. Pat. No. 5,506,925 Vengsarkar's method is applicable only to germano-silicate waveguides.
While the prior art has increased the refractive index differential between the core and cladding of a single waveguide structure formed from one particular material, there is no method for selecting an appropriate wavelength that decreases the refractive index differential in a wide variety of waveguides having different core and cladding compositions.